The tumor suppressor gene, p53, exerts a critical role in directing either cellular growth arrest or cell death (apoptosis) in response to cell stress or DNA damage. The failure of p53 to operate successful can lead to carcinogenesis. As p53 mutations are found in greater than 50% of all human cancers, understanding how p53 regulates cell growth is of high importance. p53-mediated apoptosis is a key mechanism by which an organism can eliminate a damaged and potentially tumorigenic cell from the body. The mechanism by which p53 directed apoptosis is not well understood. We have recently identified a novel regulatory protein called PW1. PW1 participates in the TNF signal transduction pathway leading to activation of the cell survival factor, NFkappaB. PW1 also interacts with the Siah family of proteins. We have identified that the Siah proteins also regulate NFkappaB activation which defines a novel role for this gene family. We have identified that the Siah proteins also regulate NFkappaB activation which defines a novel role for this gene family. We have identified PW1 as specifically induced by p53 in the context of cell death. Siahla, one member of the Siah family, has been independently identified as a p53 inducible gene. The fact that PW1 and Siah1a are activated in the context of p53, and that these genes encode proteins that subsequently interact suggested that they participate together in a p53 cellular response. Neither gene alone induces apoptosis, however, co- expression of PW1 and Siah1a leads to a dramatic induction of cell death. The involvement of PW1 in the NFkappaB (anti-apoptotic) and p53- mediated apoptotic pathways suggested a role in both cell death and survival. The aims of this project are to understand how these two opposing pathways are regulated and if NFkappaB activation can play a pro-apoptotic role. We will use cell culture models to identify how PW1 and Siah regulate cell survival/apoptosis as well as determine the contribution of the NFkappaB activation pathway in the p53 cell death program. We will also use molecular screens to identify additional participants that are required for cell death/NFkappaB activation. Lastly, we will generate transgenic mice, expressing dominant negative forms of the PW1 and Siah genes to identify their role(s) in vivo. This project will shed light on the mechanism of p53 mediated apoptosis and the role of these genes during carcinogenesis. This proposal is a result of the close collaboration of two independent research groups studying development and carcinogenesis respectively. The combined efforts bring new approaches to elucidating a fundamental question in cancer biology.